1. Field of the Invention
This invention concerns a process for removing sulfur from naphtha, a petroleum product used to make fuels like gasoline. Specifically, spent resid catalyst is used to selectively hydrodesulfurize naphtha while minimizing olefin saturation.
2. Description of Related Information
It is well known that air pollution is a serious environmental problem. A major source of air pollution worldwide is the exhaust from hundreds of millions of motor vehicles due to fuel combustion. Laws and regulations have been enacted reflecting the need to reduce harmful motor vehicle emissions through more restrictive fuel standards. Fuels containing sulfur produce sulfur dioxide and other pollutants leading to a host of environmental concerns, such as smog and related health issues, acid rain leading to deforestation, water pollution, as well as other environmental problems. To help reduce or eliminate these environmental problems, the sulfur content of fuels has been, and will continue to be, restricted to increasingly smaller concentrations, such as less than 100 or even 50 parts per million (ppm).
The problem of sulfur in fuels is compounded in many areas where there is diminishing or no domestic source of crude oil having relatively low sulfur content. For example, in the United States the supply of domestic oil production relies increasingly on lower grade crude oil with higher sulfur content. The need for lower sulfur content fuel therefore increases demand for imported oil having lower sulfur content increasing trade imbalance and vulnerability due to dependence on foreign sources of oil.
The sulfur content in crude oil can take the form of a wide variety of both aliphatic and aromatic sulfurous hydrocarbons. Various techniques have been developed for removing sulfur compounds. One such technique, called hydrodesulfurization (HDS), involves catalytically reacting hydrogen with the sulfur compounds. The general HDS reaction is illustrated in Equation 1. EQU RSR'+H.sub.2 .fwdarw.RH+R'H+H.sub.2 S EQU Equation 1: Hydrodesulfurization Reaction
In Equation 1, the sulfur compound, RSR', may be: a thiol or mercaptan, where R is hydrocarbyl and R' is hydrogen; a sulfide or disulfide, where the sulfur is connected to another sulfur atom in R or R' hydrocarbyl groups; or may be a thiophene where R and R' are connected to form a heterocyclic ring. The HDS reaction consumes hydrogen (H.sub.2) and produces hydrogen sulfide (H.sub.2 S) and hydrocarbons wherein the sulfur atom is replaced by two hydrogen atoms. The hydrogen sulfide can then be separated to give a petroleum product in which the sulfur is significantly reduced or substantially eliminated.
Petroleum refining includes the treatment of residual petroleum fractions, also called resid or bottoms, which are mixtures of higher boiling point hydrocarbons remaining after distilling lower boiling point materials from crude petroleum, in order to produce more valuable material from these residues by processes known as resid upgrading. Resid upgrading typically uses catalyst and heat at high hydrogen pressures to reform or convert, such as by catalytic hydroprocessing, residues to lower molecular weight and/or lower boiling point hydrocarbons, combined with desulfurization and demetallation.
Various resid upgrading catalysts, i.e. resid catalysts, have been described and/or used. See, for example, The Desulfurization of Heavy Oils and Residua, by J. G. Speight, Chapter 7, Marcel Dekker, New York (1981) and the Oil and Gas Journal, Oct. 14, 1991, pages 43-78.
Substantial amounts of carbonaceous, metal and/or other materials are generally deposited from the resid material onto the catalyst during its use. These deposits accumulate within the pores of the catalyst support decreasing pore volume and significantly reducing the surface area of the catalyst, resulting in diminished catalyst activity. These deposits cause the resid catalysts to lose upgrading activity producing spent resid catalyst. Spent resid catalyst generally has relatively small pore volume, typically about 0.2 cubic centimeters per gram (cc/g), and a highly reduced surface area, typically of about 50 square meters per gram (m.sup.2 /g), as compared to a pore volume of about 0.8 cc/g and a surface area of about 300 m.sup.2 /g for fresh resid catalyst. Due to the large amounts and kinds of deposits it is generally not economically viable to regenerate spent resid catalysts. Additionally, due to high, including toxic, metallic content, spent resid catalysts are frequently classified as hazardous waste and are difficult and expensive to dispose of in an environmentally sound manner, such as for metals recovery or destruction of other toxic material.
Spent hydrotreating catalysts have been used in hydrotreating, including HDS. For example, U.S. Pat. No. 3,876,532 (Plundo et al.) discloses a process for hydrotreating middle distillate, virgin oils using spent hydrotreating catalyst under extremely mild conditions to reduce acid and mercaptan content, to remove sulfur below 0.2 weight percent, or 2,000 ppm. U.S. Pat. No. 4,414,102 (Rankel et al.) discloses the use of spent HDS catalyst to transform nitrogen- or oxygen-containing compounds to sulfur-containing compounds followed by mild HDS treatment.
It would be desirable to have a process for removing sulfur from fuel feedstocks like naphtha, containing olefins, which minimizes loss of octane value, using an inexpensive procedure, to provide a cleaner environment along with a more stable economy.